The present invention relates to a method of treating particles. Specifically, the present invention contemplates treating particles so that they are separated from a liquid mass which is less dense than and wets the particles, and, more particularly, to a method of treating particles, such as spherical or nearly spherical (spheroidal) semiconductor particles, after they have been exposed to an etchant to render them the same size and suitable for inclusion in relevant products, by removing the particles from the etchant to terminate its effect on the particles. In specific embodiments, the particles are silicon spheres or spheroids and the products are photovoltaic solar cells.
While many types of solar cells are known, a type of particular interest herein includes a plurality of spherical or spheroidal semiconductor particles or members which protrude partially beyond, and are affixed to the walls of, apertures formed in a first flexible metal foil sheet. The details of the construction (i.e., the mechanical and electrical form, fit and function) and fabrication methodology of this type of solar cell are disclosed in the following commonly assigned U.S. Pat. Nos. 5,192,400; 5,091,319; 5,086,003; 5,028,546; 4,994,878; 4,992,138; 4,957,601; 4,917,752; 4,872,607; 4,806,495; and 4,691,076.
The construction details and the fabrication methodology of the type of solar cells to which the present invention may relate are here briefly summarized. First, generally uni-sized spheres or spheroids, or similar particles, of a semiconductor material, such as silicon, each having a p-n junction are produced, for example, by the production methods disclosed in commonly assigned U.S. Pat. Nos. 4,637,855; 5,012,619; or 5,069,740. The particles are typically constituted of an outer silicon portion of one conductivity type surrounding an inner silicon portion of another conductivity type, both portions having a selected purity and other relevant characteristics. The particles are capable of producing electricity when radiation, such as solar radiation, is incident thereon. The produced electricity may flow between conductors, one of which is electrically continuous with a portion of each sphere of one conductivity type, and the other of which is electrically continuous with a portion of each sphere of the other conductivity type. In the afore-noted patents, these conductors are preferably flexible metal foils, to the first of which the spheres are affixed, as noted above.
Typical silicon particle production techniques tend to produce batches of intermingled silicon spheres or spheroids having varying sizes and diameters. Thus, manufacturing the above type of solar cell, which preferably utilizes semiconductor particles (spheres and spheroids) of the same size, requires either a method of rendering the spheres the same size, as described in commonly assigned U.S. patent application, Ser. No. 160,020, filed Dec. 1, 1993 or a non-damaging mechanical method of sorting the fragile silicon spheres, such mechanical sorting methods being the subject of the following commonly assigned U.S. patent applications: Ser. No. 08/159,645, filed Nov. 30, 1993 ; Ser. No. 08/159,637, filed Nov. 30, 1993; and Ser. No. 08/159,872, filed Nov. 30, 1993. Whether uni-sized particles are achieved by a positive treatment step, as is the case with the '020 application, or by mechanical sorting, the manner of acquiring the particles should be efficient and have high throughput so as not to constitute a bottleneck in a solar cell manufacturing operation.
A contemplated method for manufacturing solar cells begins with forming in the first metal foil sheet a pattern of apertures, the diameters of which are slightly less than the diameters of an available quantity of same-sized particles. Several methods for forming the apertures are available and include first embossing and then etching the foil sheet or punching the sheet. After formation of the aperture pattern, the spheres are loaded onto the foil so that each aperture is occupied by a particle. Because of the relative sizes of the diameters of the same-sized particles and the apertures, the aperture-located particles merely nest in their respective apertures without substantially extending therethrough.
The particles are mechanically and electrically affixed and connected to the first foil. Such affixation and connection is achieved by applying suitable compressive forces to the foil-particle system, as set forth in the above-noted patents. Typically the application of the compressive forces is achieved by the use of a press which acts on the particles and the foil through selected compliant and rigid elements which are positioned between working surfaces of the press and the foil-particle system. These elements prevent damage to the particles and to the foil, while ensuring that the applied forces effectively move the particles partially through their respective apertures.
Partial movement of the particles through their respective apertures effects mechanical affixation thereof to the walls of their apertures and renders their outer surfaces electrically continuous with the first foil. These ends are achieved, in part, through the relationship of the larger diameters of the particles to the smaller diameters of the apertures. This relationship results in the mechanical and electrical affixation and aids in effecting the electrical continuity of the particles with the first foil. When the spheres are moved partially through their apertures, the edges of the aperture walls and the surfaces of the particles mechanically interact and mutually abrade each other to remove any natural oxide on either thereof. Thus, a metal-particle (i.e., an aluminum-silicon) bond is formed. The foregoing may be enhanced by the application of heat during the compression.
The outer portion of one conductivity type of the located and affixed particles is removed, as by etching. This removal occurs only on one side of the first foil to expose the inner particle portions of the opposite conductivity type. An electrically insulative layer is applied to or located on the exposed inner particle portions and the one foil side. Small regions of the layer which overlie the exposed inner particle portions are removed, as by abrading or etching, to create vias or openings which provide access to the inner particle portions. A second flexible metal foil is mechanically and electrically connected to the inner portions of the particles through conductive members in the vias by thermo-compression bonding or a functionally equivalent technique.
Radiant energy directed toward the free surface of the first foil falls on the particles which produce electricity. A utilization device is connected between the foils. The electricity flows from one portion, inner or outer, of the particles through one of the foils, through the utilization device and ultimately through the other foil into the other portion, outer or inner, of the particles. The insulative layer electrically insulates the foils from each other. The flexible cell may be conformed to a desired surface or shaped in a selected fashion. A protective cover may be placed over or applied to the spheres. The cover may include or comprise lenses which direct an increased amount of incident radiant energy onto the spheres to increase the efficiency of the cell.
In the past uniform particles diameters have been achieved by mechanical sorting followed by grinding or abrading oversized particles until they have the same diameter as spheres determined to be acceptable as a result of mechanical sorting. At least the grinding portion of a sorting-grinding methodology requires an abundance of time which affects the throughput of the overall solar cell manufacturing process. The '020 application describes an alternative etching method for treating over-sized particles so as to produce plural, same-sized particles.
Specifically, in the '020 application, following production of commingled silicon particles of various sizes, particles of sizes appropriate for inclusion in a solar cell may be separated therefrom leaving a quantity of oversized particles. The oversized particles are placed on a support surface, such as a screen, mesh or other apertured member. The apertures permit the passage therethrough, of particles having the proper size. Each particle is supported by a portion of the support surface which immediately surrounds a respective aperture so that each particle is superjacent to an aperture. The particles are sprayed with an etchant which passes through the support surface. When the etchant reduces an oversized particle to the proper size, the particle falls through the aperture into a collection zone. The support surface may be vibrated during etching to ensure that portions of all particles are subjected to the etchant and to aid a properly sized particle in falling through its aperture.
The collection zone includes particles and etchant. It is desirable to both terminate the effects of the etchant on the now properly sized particles and separate the properly sized particles from the etchant, following which the particles may be rinsed and dried. After rinsing and drying, the particles are ready to join those particles originally having sizes suitable for inclusion in a solar cell.
A general goal of the present invention is the provision of a method of separating particles from a body of fluid in which they are immersed, and a specific goal thereof is an efficient, high throughput, non-damaging method of treating same-sized semiconductor particles, which particles are suitable for use in fabricating a solar cell following their reduction in size by exposing them to an etchant, to separate the cells from the etchant.